I don't follow.
A creature, such as [CREATURE:NYMPH] has the [BODY_DETAIL_PLAN:STANDARD_MATERIALS].
[BODY_DETAIL_PLAN:STANDARD_MATERIALS], in b_detail_plan_default.txt, has [ADD_MATERIAL:LEATHER:LEATHER_TEMPLATE].
Changing the that to [ADD_MATERIAL:LEATHER:CREATURE_MAT:ANIMAL:LEATHER] makes it use the LEATHER material defined as [CREATURE:ANIMAL]'s LEATHER material, which is LEATHER_TEMPLATE.
So, the leather is still LEATHER_TEMPLATE, but now it doesn't have the name Nymph associated to it. It doesn't have any name associated to it because [CREATURE:ANIMAL] doesn't have any names in its definition. But it is still the same leather, with all of its associated reactions, as before.
And now, Nymph Leather is no longer a stockpile choice or a choice when trying to specify the material for Leather Armor. In fact, all creatures that have [BODY_DETAIL_PLAN:STANDARD_MATERIALS] in its makeup, and do not remove it with [REMOVE_MATERIAL:LEATHER], such as [CREATURE:RHENAYA_DROW_LIZARD_GIANT], no longer have a unique leather name because it is all the same as ANIMAL's leather, which has no name.
Test: Change [BODY_DETAIL_PLAN:STANDARD_MATERIALS]'s [ADD_MATERIAL:LEATHER:LEATHER_TEMPLATE] to [ADD_MATERIAL:LEATHER:CREATURE_MAT:ANIMAL:LEATHER] in b_detail_plan_default.txt. Embark with some Shaggy Badgerdogs, which uses [BODY_DETAIL_PLAN:STANDARD_MATERIALS], build your butcher and tanner and butcher them up. You'll end up with Leather, just like before, but you won't have all those other unique leathers anywhere that you can specify the leather type.